The National League West is one of the three divisions of the National League of Major League Baseball in North America (including Canada). This Division was formed for the 1969 season when the National League expanded to 12 teams by adding the San Diego Padres and the Montreal Expos. For purpose of keeping a regular-season of 162 games, half of the teams were put into the new East Division and half into the new West Division. Within each division, the teams played 18 games each against their five division mates (90 games), and also 12 games against the teams in the opposite division (72 games), totaling 162 games.

Despite the geography, the owners of the Chicago Cubs insisted that this team be placed into the East Division along with the teams in New York City, Philadelphia, and Pittsburgh. Also, the owners of the St. Louis Cardinals wanted that team to be in the same division with their natural rivals of the Cubs. The league could have insisted on a geographical alignment like the American League did. But the owners were also concerned about what they thought would be a large imbalance in the strength of the divisions. The previous two seasons, the Cardinals, Giants, and Cubs finished 1-2-3 in the National League standings. The owners were concerned about putting those teams in the same division, thereby creating one very strong division (West) and one weak one (East). Given all of this, the owners of the Atlanta Braves and the Cincinnati Reds consented to being placed into the West Division, even though Atlanta and Cincinnati are both in the Eastern Time Zone. Hence, the West Division had teams spread all the way from the East to the Pacific Coast, and scattered over four time zones. The East Division was spread over the Eastern Time Zone and the Central Time Zone - despite the fact that the National League had six teams in the Eastern Time Zone and six teams spread between the Central Time Zone and the Pacific Time Zone.

All of this increased the traveling distances and times for all of the teams, and it also made radio broadcasting and TV broadcasts of the games more difficult to schedule. The Braves and the Reds had to travel all the way to California three times during each baseball season, and the three teams in California had to travel to Atlanta, Cincinnati, and Houston three times also. Sometimes, the trouble could be alleviated for them by playing some games in Chicago, St. Louis, or Pittsburgh on the same long road trips.

The very first baseball season of division play resulted in what might be considered by many[who?] to be two of the best pennant races in Major League baseball history. In the National League West, five of the teams battled for the divisional championship - with only the expansion team, the San Diego Padres, failing to be a contender. The remaining five teams were separated by as few as one-and-one-half games in the standings on August 18, despite the Houston Astros team having lost 20 of its first 24 games.

Before the forming of a third division in both leagues in 1994, the winners of each division competed in a best-of-five series, with the series being lengthened by two possible games in 1985 to a best-of-seven series, dubbed the "League Championship Series" to determine the winner of the league pennant. This format was to be changed in 1994, though it was not carried out until 1995 due to the 1994–95 Major League Baseball strike that started on August 12, 1994. There was the addition of two further teams in the playoffs in each league. This has led to the creation of a "Division Series" round of the playoffs, in which two best-of-five series are conducted to determine the participants of the League Championship Series. As before, the winners of each league's pennant compete in the best-of-seven World Series to determine the champion of Major League Baseball.

† - Due to the 1981 Major League Baseball strike, the season was split. Los Angeles won the first half and defeated second-half champion Houston (61–49) in the postseason.
The Cincinnati Reds had the best record in the division (66-42) overall but due to the split season did not qualify for the playoffs.

§ - Due to the players' strike starting August 12, no official winner was awarded. Los Angeles was leading at the strike.

†† - The San Diego Padres and Los Angeles Dodgers finished the 2006 season tied for first place with identical records. San Diego won the season series against Los Angeles and was awarded the tie-breaker; Los Angeles was awarded the wild-card berth. Had a team from another division won the wild card, a one-game playoff would have decided the division champion.

The wild card is given to the team in each league with the best record that did not win its division and was first introduced in 1994. The system, however, was not implemented until the following season, as a player strike prematurely ended the 1994 season. Since its implementation, three different NL West teams have won the wild card, on six different occasions.

* The Colorado Rockies played the San Diego Padres in a wild card tie-breaker game after both teams finished the season with the same record, 89–73. The Rockies defeated the Padres, 9–8, in 13 innings. A wild card tie-breaker game is still considered part of the regular season, and thus, the Rockies' win made it their 90th victory of the season.

** – Starting with the 2012 season, there will be two Wild Card winners in each league. The qualifiers will play a single-game playoff to determine who will face the top-seeded team in the National League Division Series.